The present invention relates to radiocommunication apparatus which makes communication selectively using one of frequency bands respectively allocated to two communication systems such as GSM (Global system for Mobile Telecommunications) 900 and DCS (Digital Cellular System) 1800.
As is well known, there is radiocommunication apparatus that is capable of making communication selectively using one of frequency bands respectively allocated to two communication systems. FIG. 7 shows a basic arrangement of this type of radiocommunication apparatus. The conventional radiocommunication apparatus will be described below with reference to FIG. 7.
An antenna 1 is one that is capable of transmitting and receiving signals in frequency bands A and B (hereinafter referred simply as to bands A and B) which are allocated to two radiocommunication systems, respectively.
Signals received by the antenna 1 is divided in an antenna duplexer 2. The received signal in the band A is applied to a low-noise amplifier 31, while the received signal in the band B is applied to a low-noise amplifier 32.
The received signal in the band A, after being amplified by the low-noise amplifier 31, is mixed in a first downconverter 41 with a first local signal (local oscillator signal) generated by a first frequency synthesizer 18, so that it is heterodyned to an intermediate frequency (IF).
The frequency-converted received signal in the band A is applied as a first received IF signal to a first input terminal of a switch 5.
Likewise, the received signal in the band B, after being amplified by the low-noise amplifier 32, is mixed in a second downconverter 42 with a second local signal (local oscillator signal) generated by a second frequency synthesizer 19, so that it is heterodyned to an intermediate frequency. The frequency-converted received signal in the band B is applied as a second received IF signal to a second input terminal of the switch 5.
The frequencies of the first and second local signals are adjusted so that the received signals in the bands A and B are heterodyned to the same intermediate frequency.
The switch 5, as instructed by a controller not shown, selectively feeds the first received IF signal applied to the first input terminal or the second received IF signal applied to the second input terminal to a bandpass filter 6.
The bandpass filter 6 limits the bandwidth of the input IF signal to remove noise components outside of its passband.
The band-limited IF signal is amplified in an amplifier 7 up to a level appropriate for subsequent demodulation and then demodulated in a demodulator 8. The demodulated signal is applied through a signal output terminal 10 to succeeding signal processor (not shown).
The demodulator 8 employs a continuous wave (CW) signal generated by an oscillator 9 to regenerate the intermediate frequency.
In a modulator 12, the CW signal generated by the oscillator 9 is modulated with a modulating signal supplied from the signal processor via a signal input terminal 11, producing a transmit IF signal.
The transmit IF signal, after being amplified by an amplifier 13 up to an appropriate level, is applied to a lowpass filter 14 where noise and harmonic components unwanted for transmission are removed and then applied to a switch 15.
The switch 15, controlled by the controller not shown, selectively inputs the transmit IF signal to either a first upconverter 161 or a second upconverter 162.
The first upconverter 161 mixes the transmit IF signal with a first local signal generated by the first frequency synthesizer 18 for conversion to a radiofrequency signal in the band A.
The radiofrequency signal in the band A is amplified by a first power amplifier 171 and then radiated into space through the antenna duplexer 2 and the antenna 1.
The second upconverter 162 mixes the transmit IF signal with a second local signal generated by the second frequency synthesizer 19 for conversion to a radiofrequency signal in the band B.
The radiofrequency signal in the band B is amplified by a second power amplifier 172 for radiation into space through the antenna duplexer 2 and the antenna 1.
The local signals generated by the first and second frequency synthesizers 18 and 19 have their respective frequencies set to ones required for frequency conversion of the transmit IF signal to the radiofrequency signals in the bands A and B.
The conventional radiocommunication apparatus thus arranged permits the band A or B to be used selectively for communication by means of switching control of the switches 5 and 15.
The radiocommunication apparatus, if it is a mobile, may monitor the communication quality of one of the bands to prepare for hand-over during communication on the other band or monitor the communication quality of each of the bands when it is on standby for communication.
In receiving signals in two bands, it is a general practice to switch quickly to a band which is an object of reception. In switching between receiving bands quickly, in order to avoid the effect of insufficient startup of the frequency synthesizers (18 and 19 in FIG. 7), it is required to preoperate the frequency synthesizer associated with the band whose quality is to be monitored in addition to the frequency synthesizer associated with the band that is in use or operate the two frequency synthesizers from the beginning.
However, operating the two frequency synthesizers results in an increase in power dissipation. In particular, this is a serious problem for mobiles subject to constraints in continuous operating time. This problem likewise arises in the case where each of two transmission systems has its corresponding frequency synthesizer.
In addition, this problem is existent not only in such single superheterodyne radiocommunicatior apparatus as shown in FIG. 7 but also in double superheterodyne radiocommunication apparatus or TDD (Time Division Duplex)-based radiocommunication apparatus that is equipped with two frequency synthesizers for simultaneous reception of signals in two bands.
It is an object of the present invention to provide radiocommunication apparatus which permits two receiving systems to be operated simultaneously or to be switched at a high speed without any increase in power dissipation of frequency synthesizers.
To attain the object, the radiocommunication apparatus of the present invention heterodynes a received RF signal in a first frequency band allocated for a first radiocommunication system to an intermediate frequency using a signal obtained by dividing the frequency of a local oscillator signal by a factor of N and heterodynes a received RF signal in a second frequency band allocated for a second radiocommunication system to an intermediate frequency using the local oscillator signal. That is, received signals in the two frequency bands allocated for the two radiocommunication systems are heterodyned using local signals generated from a single local oscillator.
Accordingly, according to this radiocommunication apparatus, the use of only one local oscillator allows the reception (transmission) of signals in two bands while switching therebetween at high speed or the simultaneous reception (transmission) thereof without any increase in power dissipation.
Moreover, to attain the object, in the radiocommunication apparatus of the present invention, a first local oscillator signal is generated by a local oscillator and a second local oscillator signal is generated by multiplying the frequency of the first local oscillator signal by a factor of N (N greater than 0). A received signal in a first frequency band allocated for a first radiocommunication system is heterodyned using the first local oscillator signal and a received signal in a second first frequency band allocated for a second radiocommunication system is heterodyned using the second local oscillator signal. That is, the received signals in the two frequency bands allocated for the two radiocommunication systems are heterodyned using local signals generated from a single local oscillator.
Accordingly, according to this radiocommunication apparatus, the use of only one local oscillator allows the reception (transmission) of signals in two bands while switching therebetween at high speed or the simultaneous reception (transmission) thereof without any increase in power dissipation.